Variables

FQL supports two kinds of variable declarations:

let declares an immutable variable.
var declares a mutable variable that can be reassigned later.

let

The let statement assigns the result of an expression to a variable.

The variable is introduced into the scope where the let statement appears.

example.fql

read-only

let variableName = expression

After a variable is declared with let, it cannot be reassigned.

example.fql

read-only

let a = [1, 2, 3] // initial assignment a = arrays::append(a, 4) // error: a cannot be reassigned let a = arrays::append(a, 4) // error: a is already declared in this scope let b = arrays::append(a, 4) // allowed, result: [1, 2, 3, 4]

arrays::append returns a new array and leaves a unchanged. The first two lines after the initial binding intentionally demonstrate invalid assignments.

let bindings often appear where a query needs to refer to an intermediate value, a subquery result, or another computed expression by name.

example.fql Ferret v2

query.fql

FQL

let users = [ { "id": 1, "firstName": "Kikelia", "lastName": "Coper", "cart": [ { "Name": "Garlic", "Price": "$7.46" }, { "Name": "Flower - Commercial Spider", "Price": "$6.59" } ] }, { "id": 2, "firstName": "Toni", "lastName": "MacTeggart", "cart": [ { "Name": "Spice - Paprika", "Price": "$6.31" }, { "Name": "Extract - Vanilla,artificial", "Price": "$4.74" }, { "Name": "Wine - White, Cooking", "Price": "$1.50" }, { "Name": "Nutmeg - Ground", "Price": "$1.30" } ] }, { "id": 3, "firstName": "Neile", "lastName": "Saice", "cart": [ { "Name": "Mustard Prepared", "Price": "$2.28" }, { "Name": "Flower - Commercial Bronze", "Price": "$4.80" } ] } ] return for u in users { let numProducts = length(u.cart) return { "user": u, "numProducts": numProducts, "discount": numProducts >= 3 } }

In this example, numProducts stores the number of items in the user’s cart. The value can then be reused in the returned object without calling length(u.cart) more than once.

let is also useful for assigning the result of a subquery to a variable.

example.fql Ferret v2

query.fql

FQL

let users = [ { "id": 1, "name": "Moises Grisewood" }, { "id": 2, "name": "Dell Marnes" }, { "id": 3, "name": "Tobin Bilbery" }, { "id": 4, "name": "Lorianne Posten" }, { "id": 5, "name": "Drucill Cryer" } ] let friends = [ { "id": 1, "name": "Maximo Massard", "userId": 1 }, { "id": 2, "name": "Delainey Sancho", "userId": 1 }, { "id": 3, "name": "Lindon Beale", "userId": 1 }, { "id": 4, "name": "Gus Sprey", "userId": 3 }, { "id": 5, "name": "Virgil Dallander", "userId": 3 }, { "id": 6, "name": "Agretha Mackerel", "userId": 4 }, { "id": 7, "name": "Christalle Aldins", "userId": 4 }, { "id": 8, "name": "Karalynn Margery", "userId": 5 }, { "id": 9, "name": "Rodolph Ladd", "userId": 5 }, { "id": 10, "name": "Babette Brassill", "userId": 1 } ] return for u in users { let friends = ( for f in friends { filter u.id == f.userId return f } ) return { "user": u, "friends": friends, "numFriends": length(friends) } }

Here, the inner for query finds the friends that belong to the current user. Assigning the result to friends makes the return statement easier to read and allows the same value to be used more than once.

var

The var statement declares a mutable variable. Unlike variables declared with let, variables declared with var can be reassigned.

The general syntax is:

example.fql

read-only

var variableName = expression

A var variable can be updated later using assignment syntax:

example.fql Ferret v2

query.fql

FQL

var count = 0 count = count + 1 count = count + 1 return count

var is suitable for values that change across multiple statements, such as counters, accumulators, or flags.

example.fql Ferret v2

query.fql

FQL

let numbers = [1, 2, 3, 4, 5] var total = 0 return for n in numbers { total = total + n return total }

In this example, total starts at 0 and is updated for each value in numbers.

Destructuring declarations

let and var can bind several names from one object or array. The source expression is evaluated once.

example.fql Ferret v2

query.fql

FQL

let { name, profile: { city }, scores: [first, _, third] } = { name: "Ada", profile: { city: "London" }, scores: [10, 20, 30] } return { name, city, first, third }

Object entries use the property name as the binding name. Add : to use a different name or a nested pattern. Array entries are positional, and _ explicitly ignores a property or position without reading it. A nested child pattern with no named bindings, such as metadata: [_, _] or details: {}, is ignored as a whole: Ferret does not retrieve or validate that child value. Patterns can be nested recursively and can end with a trailing comma. Empty patterns ({} and []) are valid.

Every named leaf follows the declaration kind. Leaves declared by let are immutable. Leaves declared by var are independently mutable:

example.fql Ferret v2

query.fql

FQL

var { count, step } = { count: 1, step: 2 } count += step step += 1 return { count, step }

Missing properties or elements bind none. Destructuring none also propagates none through nested patterns, while extra source values are ignored. A non-none value reached by the root pattern or by a child pattern needed to produce a binding must support keyed access for an object pattern or indexed access for an array pattern; otherwise execution fails with cannot destructure <Actual> as Object or cannot destructure <Actual> as Array. Explicit empty root patterns still perform this shape check; ignored child patterns do not.

Array holes are not allowed; write _ for each ignored position. Defaults, rest or spread entries, quoted or computed object keys, and conditions are not supported in binding patterns. Destructuring is declaration syntax only: assignments and function parameters still bind one existing target or name.

Compound assignment

FQL supports compound assignment operators that combine an arithmetic operation with assignment. They are shorthand for updating a var variable in place:

Operator	Equivalent
`a += b`	`a = a + b`
`a -= b`	`a = a - b`
`a *= b`	`a = a * b`
`a /= b`	`a = a / b`

example.fql Ferret v2

query.fql

FQL

var total = 100 total += 50 total -= 10 total *= 2 return total

Compound assignment operators can only be used with var variables. Using them with let bindings is an error.

Next steps

Learn Expressions Build values from literals, operators, and function calls. Learn Operators Compare, combine, and transform values with FQL's operators. Learn Control Flow Control the flow of your script with conditional statements, loops, and subqueries.